Polycaprolactone (PCL) is a polymer capable of microbiological degradation and various applications have been proposed for PCL as an environment friendly polymer. However, the relatively low melting point (60.degree. C.) of PCL has limited its applicability to a wider scope of fields. According to the present invention, PCL is melted at 60.degree. C. or higher and then cooled down to a non-crystallizing temperature, at which it is exposed to a radiation to introduce a network structure through crosslinking, thereby imparting heat resistance to the PCL. The irradiated PCL has the potential to be used as a tape or a heat shrinkable film.
Radiations used for engineering purposes include gamma-rays from cobalt 60 and electron beams from an accelerator. Having great penetrating power, gamma-rays are mainly used to sterilize medical devices which are final goods packed in corrugated board boxes. The penetrating power of electron beams is not as great as that of gamma-rays but they have the advantage of allowing for continuous application within a short time. Hence, electron beams are extensively used in modifying high-molecular weight materials through crosslinking, graft polymerization or decomposition reaction. The use of electron accelerators is more active than any other types of accelerators since by exposure to accelerated electron beams, a crosslinked structure can be easily introduced into high-molecular weight materials so as to achieve a marked improvement in their heat resistance. In the field of electrical wire coatings made from polyethylene and polyvinyl chloride, electron accelerators have been used for quite many years in order to improve their heat resistance through crosslinking. Several of such high-molecular weight materials require high doses radiation to be crosslinked and, hence, a technique has been proposed that allows for the crosslinking reaction to take place at lower doses through the addition of reactive monomers and the like.
However, not all of the reactive monomer used is consumed in the crosslinking reaction and it remains unreacted in a small amount; hence, the technique is held to be unsuitable for application to materials that are to be used in contact with food.
High-molecular weight materials are composed of either a crystalline region or a non-crystalline region. Since crosslinking due to irradiation mainly occurs in the non-crystalline region, some polymers such as polyethylene will readily crosslink if they are heated to temperatures higher than the melting point and exposed to radiation in an amorphous state. In polytetrafluoroethylene (Teflon), a crosslinking reaction does not occur at room temperature but a decomposing reaction will predominate; according to a recent finding, a crosslinking reaction occurs preferentially in polytetrafluoroethylene in a certain temperature range exceeding its melting point.
In order to crosslink PCL by irradiation at temperatures near room temperature, a dose as high as 200 kGy is required and yet a maximum gel fraction (a measure of the degree of crosslinking) that can be achieved is about 25%. Hence, no adequate improvement in the heat resistance of PCL cannot be accomplished at temperatures near room temperature. On the other hand, crosslinking is more likely to occur in PCL at temperatures near its melting point; however, the crosslinked product formed after the irradiation contains so many voids that a film compression molded from it has only low strength.
Under the circumstances, the present inventors conducted intensive studies and found that when PCL melted at 60.degree. C. was cooled down to a non-crystallizing temperature and then exposed to radiations at that temperature, a product of an extremely high gel fraction was obtained at a lower dose than required by irradiation at temperatures near room temperature or the melting point of PCL; it was also found that the product could be compression molded with a hot press into a film having heat resistance and a high degree of transparency.